It is known in antenna systems to have two parallel coaxial lines extending vertically upwards along the tower. These coaxial lines each include, for example, up to 2,000 feet or more of coaxial tubing in sections, forming a coaxial line fixed to the tower at the top of the line, so that the line is suspended from its top end.
Both coaxial lines may be suspended at points along their length by spring hangers from the tower to allow the coaxial lines to expand and contract with respect to the tower. The spring hangers provide stability while permitting vertical travel of the line relative to the tower due to factors such as thermal expansion of the line relative to the tower. Temperature variations produced by weather and the operating power of the coaxial line cause the coaxial lines to expand at a different rate than the tower. The coaxial line and the tower are also made of different materials, further contributing to differential expansion between the lines and the tower. For example, the coaxial line may be made of copper and the tower made of steel. Since these two metals have different coefficients of expansion, there is a differential in the thermal growth of the copper coaxial line with respect to the steel tower as temperature and power changes.
For this reason, it is known to suspend the coaxial lines from the top of the tower, so they are fixed both vertically and horizontally at the top of the coaxial line to the tower, but are essentially hanging in a suspended state from the top, with the lines being horizontally restrained by spring hangers that permit vertical movement along the length of the line. This permits the length of the line to have vertical travel, and the lower end of the coaxial lines, which usually terminate in an elbow connecting to a horizontal coaxial line section, are free to travel vertically relative to the tower.
A disadvantage of the known arrangement is that one of the two parallel coaxial lines may expand at a different rate than the adjacent coaxial line. For example, if one coaxial line is heated by the sun and the other coaxial line is in the shade, the first coaxial line will expand at a different rate than the second coaxial line. The differential in the relative linear expansion between two adjacent coaxial lines can cause a phase difference in the transmission of signals transmitted through the lines, which can result in undesirable beam tilt when the signal reaches the antenna. That is, if the two coaxial lines expand by different degrees along their length, the distance from the lower elbow to the fixed top portion of the line for each line will be a different total distance. Therefore, if one line elongates more than the other adjacent line, the effective and actual transmission length of the two lines will be different. Because these two lines are intended to carry signals that are at a fixed relative phase at the elbows in the lower portion of the lines, the change in length is undesirable because the signals at the top of the coaxial lines will become out of phase due to their having traveled a different distance.
Accordingly, there is a need for an arrangement that can tie together a pair of parallel coaxial lines and accommodate for differential expansion between sections of the adjacent lines while maintaining a constant relative total length between two points of the lines, such as for example, between a lower elbow and a fixed top end of each line.